An implantable device, such as an implantable cardiac rhythm management device (e.g., a pacemaker, a defibrillator, or a cardioverter), may be used to monitor cardiac function and provide therapy for a patient who suffers from cardiac arrhythmia. For example, in an attempt to maintain regular cardiac rhythm, the implantable device may track the type and timing of native cardiac signals. In this way, the implantable device may determine whether cardiac events (e.g., contractions) are occurring and whether they are occurring at the proper times.
The implantable device may track cardiac signals through the use of one or more leads implanted in or near the heart of the patient. For example, the implantable device may process signals received via implanted leads and then attempt to characterize the received signals as a particular cardiac event. Such cardiac events may include, for example, P waves, R waves, T waves, or arrhythmia events. By analyzing the type and timing of these cardiac events, the implantable device may determine whether therapy should be provided and, if so, the type of therapy to be provided (e.g., stimulation pulses).
For example, pacemakers typically employ one or more intravascular leads that connect to a so-called “can” containing a battery and associated electronics for pacing and sensing. Single-chamber pacemakers in the right atrium (RA) or right ventricle (RV) would typically be programmed in AAI or VVI modes, respectively, to inhibit pacing whenever intrinsic activity in that chamber is detected.
A dual-chamber pacemaker with RA and RV leads or a dual-chamber lead may have the ability to sense both atrial and ventricular electrical activity. For any patient with intermittent AV node conduction, it may be preferable to inhibit ventricular pacing and allow an intrinsic R wave to occur for a time after any P wave is detected on the RA lead. If ventricular pacing is needed, it is desirable to synchronize ventricular activity to atrial activity using an AV delay. The VDD programming mode has become common in dual-chamber pacemakers for patients with various degrees of AV block. Other common dual chamber modes include DDD and DDDR.
Current implantable medical devices (IMDs) for cardiac applications, such as pacemakers, include a “housing” or “can” and one or more electrically-conductive leads that connect to the can through an electro-mechanical connection. The can is implanted outside of the heart, in the pectoral region of the patient and contains electronics (e.g., a power source, microprocessor, capacitors, etc.) that provide pacemaker functionality. The leads traverse blood vessels between the can and heart chambers in order to position one or more electrodes carried by the leads within the heart, thereby allowing the device electronics to electrically stimulate or pace cardiac tissue and measure or sense myocardial electrical activity.
To sense atrial cardiac signals and to provide a right atrial chamber stimulation therapy, the can is coupled to an implantable right atrial lead including an atrial tip electrode that typically is implanted in the patient's right atrial appendage. The right atrial lead may also include an atrial ring electrode to allow bipolar stimulation or sensing in combination with the atrial tip electrode.
Before implantation of the can into a subcutaneous pocket of the patient, however, an external pacing and measuring device known as a pacing system analyzer (PSA) is used to ensure adequate lead placement, maintain basic cardiac functions, and evaluate pacing parameters for an initial programming of the device. In other words, a PSA is a system analyzer that is used to test how the leads would perform with an implantable device, such as an implantable pacemaker.
To sense right ventricular cardiac signals and provide ventricular stimulation therapy, the can is coupled to an implantable right ventricular lead including a right ventricular (RV) tip electrode and a right ventricular ring electrode. The lead for an implantable cardioverter defibrillator may also contain one or more electrodes for delivery of high-voltage therapy, such as a right ventricular coil electrode, a superior vena cava (SVC) coil electrode, or both. Typically, the right ventricular lead is transvenously inserted into the heart so as to place the right ventricular tip electrode in the right ventricular apex such that the RV coil electrode is positioned in the right ventricle and the SVC coil electrode is positioned in the right atrium and/or superior vena cava. Accordingly, the right ventricular lead is capable of receiving cardiac signals, and delivering stimulation in the form of pacing and shock therapy to the right ventricle.
A pacing system may also deliver resynchronization therapy, in which electrical stimulation is delivered to coordinate the electromechanical activity of the chambers of the heart. Such a pacing system may use the leads placed in the right atrium and right ventricle along with an additional lead coupled to the can that extends through the coronary sinus to a distal tip electrode on the outer surface of the left ventricle. There may be one or more ring electrodes in electrical contact with the left ventricle, the left atrium or both. The tip electrode may reach a location in the venous vasculature of the left ventricle, including any portion of the coronary sinus, great cardiac vein, left marginal vein, left posterior ventricular vein, middle cardiac vein, and/or small cardiac vein, or any other cardiac vein accessible by the coronary sinus.
Accordingly, the coronary sinus lead may be designed for some or all of the following functions: receive atrial and/or ventricular cardiac signals; deliver left ventricular pacing therapy using a left ventricular tip electrode for unipolar configurations or in combination with at least one left ventricular ring electrode for bipolar configurations; deliver left atrial pacing therapy using at least one left atrial ring electrode; deliver high-voltage therapy using at least one coil electrode.
Although a portion of the leads are located within the heart, a substantial portion of the leads, as well as the can itself are outside of the patient's heart. Consequently bacteria and the like may be introduced into the patient's heart through the leads, as well as the can, thereby increasing the risk of infection within the heart. Additionally, because the can is outside of the heart, the patient may be susceptible to Twiddler's syndrome, which is a condition caused by the shape and weight of the can itself. Twiddler's syndrome is typically characterized by subconscious, inadvertent, or deliberate rotation of the can within the subcutaneous pocket formed in the patient. In one example, a lead may retract and begin to wrap around the can. Also, leads may dislodge from the endocardium or veins and cause the device to malfunction. Further, in another typical symptom of Twiddler's syndrome, the device may stimulate the diaphragm, vagus, or phrenic nerve, pectoral muscles, or brachial plexus. Overall, Twiddler's syndrome may result in sudden cardiac arrest due to conduction disturbances related to the device.
In addition to the foregoing complications, implanted leads may experience certain further complications such as incidences of venous stenosis or thrombosis, device-related endocarditis, lead perforation of the tricuspid valve and concomitant tricuspid stenosis; and lacerations of the right atrium, superior vena cava, and innominate vein or pulmonary embolization of electrode fragments during lead extraction.
To combat the foregoing limitations and complications, small sized devices configured for intra-cardiac implant have been proposed. These devices, termed leadless pacemakers (LLPM), are typically characterized by the following features: they are devoid of leads the pass out of the heart to another component, such as a pacemaker can outside of the heart; they include electrodes that are affixed directly to the can of the device; the entire device is attached to the heart; and the device is capable of pacing and sensing in the chamber of the heart where it is implanted.
LLPM devices that have been proposed thus far offer limited functional capability. These LLPM devices are able to sense in one chamber and deliver pacing pulses in that same chamber, and thus offer single chamber functionality. For example, an LLPM device is located in the right atrium would be limited to offering AAI mode functionality. An AAI mode LLPM can only sense in the right atrium, pace in the right atrium and inhibit pacing function when an intrinsic event is detected in the right atrium within a preset time limit. Similarly, an LLPM device that is located in the right ventricle would be limited to offering VVI mode functionality. A VVI mode LLPM can only sense in the right ventricle, pace in the right ventricle and inhibit pacing function when an intrinsic event is detected in the right ventricle within a preset time limit. To gain widespread acceptance by clinicians, would be highly desirable for LLPM devices to have dual chamber pacing/sensing capability (VDD or DDD mode) along with other features, such as rate adaptive pacing.
It has been proposed to implant sets of multiple LLPM devices within a single patient, such as when one or more LLPM devices located in the right atrium and one or more LLPM devices located in the right ventricle. The atrial LLPM devices and the ventricular LLPM devices wirelessly communicate with one another to convey pacing and sensing information there between to coordinate pacing and sensing operations between the various LLPM devices.
However, these sets of multiple LLPM devices experience various limitations. For example, if there is a wireless communication link between the devices, both devices must expend power to maintain the link. The wireless communication link should be maintained continuously in order to constantly convey pacing and sensing information between, for example, atrial LLPM device(s) and ventricular LLPM device(s). This exchange of pacing and sensing information is necessary to maintain continuous synchronous atrioventricular coordination.
Further, it is difficult to maintain a reliable wireless communications link between LLPM devices. The LLPM devices utilize low-power transceivers that are located in a constantly changing environment within the associated heart chamber. The transmission characteristics of the environment surrounding an LLPM device change due in part to the continuous cyclical motion of the heart and change in blood volume. Hence, the potential exists that the communication link is broken or intermittent.